1. Technical Field
This invention relates to an isolated switching power source in which the ground is separated in an input side and an output side of the power source.
2. Description of the Prior Art
In an isolated switching power source, in order to stabilize an output voltage for a voltage change of an input power source or a load change, a feedback from the output side to the input side of the power source by using a photo-coupler is achieved, thereby controlling a switching transistor in the input side of the power source.
FIG. 1 shows a circuit of a conventional isolated switching power source. In FIG. 1, the circuit comprises an input power source 1, capacitors 2, 7, a transformer 3, a switching transistor 4, a control circuit 5 of the switching transistor 4, a diode 6, voltage dividing resistors 8, 9, a photo-coupler light-receiving element 10-2 incorporated with a photo-coupler light-emitting element 10-1, an error amplifier 17, a reference voltage source 18 and a resistor 19.
A primary winding and a secondary winding of the transformer 3 are of opposite polarity each other as shown in FIG. 1 so that a voltage is induced in a direction of an arrow E when the switching transistor 4 is switched from ON to OFF by the control circuit 5. After the voltage induced in the secondary winding of the transformer 3 is rectified by the diode 6, an output voltage V.sub.0 smoothed by the capacitor 7 is outputted. The output voltage V.sub.0 is divided by the voltage dividing resistors 8, 9 and the divided voltage is inputted to the error amplifier 17. To another input terminal of the error amplifier 17, the reference voltage source 18 generating a reference voltage is connected so that a current corresponding to a difference between a voltage V.sub.1 which is obtained by dividing the output voltage V.sub.0 through the voltage dividing resistors 8, 9 and the reference voltage can flow to the photo-coupler light-emitting element 10-1. It should be noted that for example, a Zener diode or a band-gap voltage source is used as the reference voltage source 18.
When the current flows to the photo-coupler light-emitting element 10-1, it can emit quantity of the light corresponding to amount of the current and provide the photo-coupler light-receiving element 10-2 with the light. As a result, a feedback signal according to a difference between the reference voltage and the voltage V.sub.1 is applied to the control circuit 5. The control circuit 5 performs ON and OFF control of the switching transistor 4 so as to maintain the constant output voltage V.sub.0 based on the feedback signal. Like this, the conventional isolated switching power source has controlled the output voltage V.sub.0.